This application is based on Application No. HEI 10-160278 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a focus detecting device for use in cameras and the like, and specifically relates to a focus detecting device capable of large amount of defocus.
2. Description of the Related Art
The phase difference detecting method is representative of conventional focus detecting methods used in cameras and the like. The phase difference detecting method is the method typically used in modern single-lens reflex cameras. The phase difference detecting method determines the focus state via the relative divergence of a secondary image formed by a separator lens comprising a pair of relay lenses arranged behind a film equivalent plane. FIG. 11 is a perspective view of a conventional autofocus module using the phase difference detecting method. In the drawing, reference number 51 refers to a filter for cutting the infrared component. Reference number 52 refers to a condenser lens. Reference number 53 refers to a mirror. Reference number 54 refers to a an aperture mask. Reference number 55 refers to a separator lens. Reference number 56 refers to a charge-coupled device (CCD) sensor.
As shown in the drawing, the filter 51 cuts the infrared component of the photographic light 50 passing through a taking lens (not shown in the drawing) and forming an image in the vicinity of the film equivalent plane. Furthermore, the light is divided into a pair of luminous fluxes in the on-axis and extra-axis focus areas of the optical axis X by the condenser lens 52 and the aperture mask 54 between which the mirror 53 is interposed. The luminous flux again forms an object image in two areas (standard area and reference area) on the CCD sensor 56 provided in correspondence to a pair of lenses via the separator lens 55.
The re-formed object image has a narrow image interval in the front focus state and a wide image interval in the rear focus state compared to the in-focus state. The focus position is detected by detecting the image interval of the object image on the CCD sensor 56.
However, the focus detecting range in the phase difference detecting method is generally approximately xc2x110 mm. When the focus detection range is wider than approximately xc2x110 mm, the distance is increased from the so-called film equivalent plane to the surface of the CCD sensor 56, thus increasing the size of the device. Furthermore, the depth of focus must be ensured to have a focal point at which the secondary image on the surface of the CCD sensor 56 corresponds to a broad focus detecting range. For this reason, a F-number value of the separator lens 55 comprising a relay lens system must be used to produce darkening, thus making use difficult in the low luminance range.
The taking lens which is a weak point of the focus detecting device using the phase difference detecting method is the macro lens. Since this lens has a high photographic magnification of near objects, large movement of focusing (amount of defocus), and an excessively large focus range on a single view of the focus detecting device, the taking lens must be driven to a position at which focus detection is possible. Under low contrast conditions, for example, a slow lens drive control is executed to drive the lens to a position at which focus detection can be achieved (hereinafter referred to as xe2x80x9clow contrast scanxe2x80x9d).
In this instance, since the focus direction is unclear, the operation may move the lens to the end in the direction opposite the focus direction and returning the lens in the focus direction. Since the macro lens has a large amount of extension, time is required to drive the lens, thereby producing a large time loss to attain focus. These disadvantages are similarly present in telephoto lenses. This tendency is particularly pronounced when a telephoto macro lens is used, thus adversely affecting the autofocus operation characteristics.
An object of the present invention is to provide a focus detecting device capable of detecting the focus direction and correspondingly capable even in the case of a large defocus which cannot be detected by a conventional focus detection device. These objects are attained by the present invention comprising an optical system which forms an image of a photographic subject, a first sensor pair having a first image sensor and a second image sensor which receives light flux from said subject through said optical system, a second sensor pair having a third image sensor and a forth image sensor which receives light flux from said subject through said optical system, a first focus detector which detects a focus condition of said optical system based on outputs of said first sensor pair, a second focus detector which detects a focus condition of said optical system based on outputs of said second sensor pair, and a third focus detector which detects a focus condition of said optical system based on outputs of said first image sensor and said third image sensor.
One image sensor among the first sensor pair and one image sensor among the second sensor pair are arranged at symmetrical positions relative to the optical axis.
A focus correcting lens is inserted directly anterior to one image sensor among the first sensor pair and one image sensor among the second sensor pair, respectively.
This construction provides a focus detecting device capable of detecting the focus direction and correspondingly capable even in the case of a large defocus which cannot be detected by a conventional focus detection device.